The present invention relates to magnetic recording media and magnetic recording apparatus for use with these media. More particularly, the invention relates to improvement to a lubricant layer formed on a protecting overcoat of a magnetic recording medium.
As well known, magnetic recording apparatus is divided into those using a magnetic disk as the medium on which to record data and those using magnetic tape as such medium. Because the former type of recording apparatus using a magnetic disk (hereinafter referred to as magnetic disk drives) is prevailing, the following description focuses on magnetic disk drives as an example of the magnetic recording apparatus.
As the capacity enlargement of magnetic disk drives has been pursued for recent years, the fly height of the magnetic head has been lowered rapidly down to below 30 nm, and, consequently, there is increasing need for reliability in terms of resistance to sliding friction.
Also, there is strong need to enhance the data processing speed with more disk capacity. In particular, in a Redundant Array of Independent Disks (RAID) system, a magnetic disk drive that operates at a disk revolving speed of 10,000 rmp or higher is required.
In order to ensure the reliability of a magnetic disk drive, generally, a lubricant layer is formed on a carbon overcoat on the surface of a magnetic disk for use in the disk drive. As the main material of the lubricant layer, usually, fluoropolyether which is a chemically stable fluorinated organic compound is widely used.
As lubricants of this type, commercial articles have been available on the market and used as disk lubricants, for example, Fomblin AM2001, Fomblin Z-DOL, Fomblin Z DOL-TX, Fomblin Z TETRAOL, etc. supplied by Ausimont (the company name changed to Solvay Slexis on Dec. 13, 2002), and Demnum SA, Demnum SP, etc. supplied by Daikin Industries, Ltd.
Meanwhile, as for fluoropolyether lubricants which are commonly used, a lubricant comprises fluoropolyether with average molecular weight of 1,000 to 10,000 as the backbone and a polar functional group as its end group.
In order to assure the reliability of a magnetic disk drive, it is necessary to preserve the lubricant on the surface of a magnetic disk used in the disk drive for long time.
However, when the magnetic disk drive operates, the disk revolves at a high speed and the lubricant is spun off from the disk by air shear due to the air flow on the surface of the disk as the disk revolves and the centrifugal force directly exerted on the lubricant. In consequence, the quantity of the lubricant on the surface of the disk gradually decreases. Moreover, it is known that a phenomenon of evaporation of the lubricant into the atmosphere inside the magnetic drive takes place.
To overcome this problem of the lubricant loss by being spun off during disk revolution and natural evaporation, approaches have heretofore been proposed that produced advantageous result to some extent, including a method using a lubricant with high molecular weight for which the average modular weight is more than 3,000 and a method of producing a lubricant excluding components with low molecular weight and forming a lubricant layer of this lubricant on the disk surface.
For example, commercial lubricant article Fomblin Z DOL supplied by Ausimont is based on the proposed method in which molecular weight distribution is controlled so that stable lubrication will be achieved for long time (refer to, for example, Japanese Patent Laid-Open Publication No. Hei 9-120524 and Japanese Patent Laid-Open Publication No. 2000-315314).
However, only by the molecular weight distribution control which is applied to Fomblin Z DOL, the lubricant cannot be sufficiently restrained from being spun off and evaporation from the magnetic disk surface for magnetic disk drives that operate at a high disk revolving speed of 10,000 rmp or higher or in hard environment such as high ambient temperature.
As one means for solving this problem, a method for restraining the lubricant from being spun off and evaporation has been proposed in which the adhesion force of the lubricant to the disk protecting layer is made stronger by intensifying the polarity of the functional end group in the lubricant (refer to, for example, Japanese Patent Laid-Open Publication No. 2001-164279).
Among the commercial lubricant articles available on the market, for example, Fomblin Z DOL-TX and Fomblin Z TETRAOL are known to have stronger adhesiveness than Fomblin Z DOL. Above all, Fomblin Z TETRAOL is known to have even stronger adhesiveness.
Meanwhile, a lubricant having higher adhesive force to the disk protecting layer is intrinsically of higher polarity and has such a problem in practical lubricant layer formation that it is hardly dissoluble in a solvent for preparing the lubricant solution when applying the solution in which the lubricant dissolved on top of the disk protecting layer and forming the lubricant layer. In order to apply a lubricant on top of the disk, solution in which the lubricant dissolved is prepared and a fluorinated solvent is generally used as the solvent for allowing a fluorinated lubricant such as fluoropolyether to dissolve in the solution.
As fluorinated solvents, the following can be mentioned: perfluorocarbon, fluoropolyether, hydrofluoroether, hydrofluorocarbon, hydrofluorocarbon including chlorine, etc. As typical examples of commercial articles of fluorinated solvents available on the market, the following are mentioned: e.g., PFC-5060 supplied by 3M, GALDEN supplied by Ausimont, HFE-7100 supplied by 3M, Vertrel XF supplied by Dupont, and ASAHIKLIN AK-225 supplied by Asahi Glass Company.
The above-mentioned commercial lubricant article, Fomblin Z TETRAOL includes dihydric alcohol functional groups at either ends, but injection of the functional groups into either ends of molecules is not completely performed. In the Fomblin Z TETRAOL, components into which monohydric alcohol functional groups are injected coexist with non-functionalized end groups in which alcohol functional groups are not injected at all.
Due to the coexistence of the dihydric and monohydric alcohol functional groups and non-functionalized end groups, for the Fomblin Z TETRAOL, normal alcohol valences fall within a range of 3.00 to 3.95 and alcohol hydration distribution which corresponds to difference between the greatest and smallest alcohol valences among the component molecules is 0.95. Moreover, the quantities of the existing dihydric and monohydric alcohol functional groups and non-functionalized end groups vary among production lots.
In a fluoropolyether lubricant, the components with a high alcohol valence have high molecular polarity, which exhibits a lubricant property of high adhesiveness to the magnetic disk protecting layer, but they are not dissoluble in a solvent such as, for example, perfluorocarbon and fluoropolyether, which is used to allow the lubricant to dissolve in solution to be applied on top of the protecting layer. Such components are not completely dissoluble in a solvent having polarity such as hydrofluoroether, hydrofluorocarbon, and hydrofluorocarbon including chlorine. This is a trouble in the production of magnetic disks.
In a fluoropolyether lubricant, on the other hand, the components with a low alcohol valence have low molecular polarity, which exhibits a lubricant property of low adhesiveness to the magnetic disk protecting layer, and they cannot well restrain the lubricant layer thickness from decreasing for magnetic disk drives that operate at a high disk revolving speed or in hard environment such as high ambient temperature.
Seeking to solve the above-described problems of prior art, it is therefore an object of the present invention is to provide a magnetic recording medium typified by a high-reliability magnetic disk having a lubricant layer with excellent lubrication properties, a method of manufacturing thereof, and a magnetic recording apparatus for use with the magnetic recording medium.
It is another object of the present invention is to provide a high-reliability magnetic disk drive, specifically, a high-density magnetic disk drive which primarily comprises a magnetic disk revolving at a high speed and a magnetic head which flies as low as possible over the magnetic disk.
In order to solve the above-described problems, the inventors conducted tests to examine lubricants comprising fluoropolyether for dissolubility in a solvent and the adhesive property of the lubricant layer to the overcoat of the magnetic disk (degree of decrease of the lubricant layer thickness).
As a result, we obtained important findings below. By controlling the alcohol valences of a lubricant comprising fluoropolyether as the principal ingredient to fall within a certain range and adjusting alcohol hydration distribution which corresponds to difference between the greatest and smallest alcohol valences among the component molecules to a certain value, the lubricant dissoluble in a solvent can be obtained and the lubricant layer formed by applying the lubricant on the surface of a magnetic disk can be well restrained from decreasing in thickness. By this method, a magnetic recording apparatus and magnetic recording medium (magnetic disk) of high reliability and high density can be realized easily.
The present invention has been made, based on the above findings, and specific features of the invention in aspects in which the objects of the invention are achieved will be described below.
According to a first aspect of the invention, the invention provides a magnetic recording medium comprising a non-magnetic substrate and, at least, a magnet layer for carrying out magnetic recording and retrieval, a protective layer, and a lubricant layer which are formed in order on the non-magnetic substrate, wherein the lubricant layer includes fluoropolyether which is represented by chemical formula (1) which is shown below and the fluoropolyether is characterized by having alcohol hydration distribution of 0.48 or less which corresponds to difference between the greatest and smallest alcohol valences among component molecules and an average alcohol valence among component molecules within a range of 3.00 to 3.91.
R1xe2x80x94CF2Oxe2x80x94[xe2x80x94(CF2CF2O)mxe2x80x94(CF2O)nxe2x80x94]xe2x80x94CF2xe2x80x94R2xe2x80x83xe2x80x83(1)
where m and n are positive integers and R1 and R2 are end functional groups respectively represented by:
xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94OH
and
xe2x80x94CH2xe2x80x94OH
Practically desirable values of m and n in the chemical formula (1) are integers which fall within a range of 3 to 50 and the average alcohol valence among component molecules falls within a range of 3.38 to 3.91. It is desirable that the protective layer is made of carbonaceous material such as, for example, diamond like carbon. Practically desirable alcohol hydration distribution of the fluoropolyether falls within a range of 0.072 to 0.48.
According to a second aspect of the invention, the invention provides a method of manufacturing a magnetic recording medium comprising the steps of depositing and forming, at least, a magnet layer, a protective layer, and a lubricant layer in order on a non-magnetic substrate. A process of forming the lubricant layer comprises the steps of preparing a lubricant which comprises fluoropolyether as the principal ingredient, wherein the fluoropolyether is represented by chemical formula (1) which is shown below, letting the lubricant dissolve in a solvent, thus making a lubricant solution, and applying the lubricant solution on top of the protective layer formed on the substrate, wherein the fluoropolyether is characterized by having alcohol hydration distribution of 0.48 or less which corresponds to difference between the greatest and smallest alcohol valences among component molecules and an average alcohol valence among component molecules within a range of 3.00 to 3.91.
R1xe2x80x94CF2Oxe2x80x94[xe2x80x94(CF2CF2O)mxe2x80x94(CF2O)nxe2x80x94]xe2x80x94CF2xe2x80x94R2xe2x80x83xe2x80x83(1)
where m and n are positive integers and R1 and R2 are end functional groups respectively represented by:
xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94OH
and
xe2x80x94CH2xe2x80x94OH
In the foregoing method of manufacturing a magnetic recording medium according to the second aspect of the invention, the step of preparing a lubricant which comprises fluoropolyether as the principal ingredient, represented by the chemical formula (1), comprises a step of fractionating and controlled extraction of fluoropolyether component molecules, using a carbon-bearing gas solvent whose density is controlled by controlling the solvent temperature and pressure by a supercritical extraction and solvent control method in which alcohol hydration distribution which corresponds to difference between the greatest and smallest alcohol valences among the component molecules of the fluoropolyether is adjusted to 0.48 or less and an average alcohol valence among the component molecules is adjusted to fall within a range of 3.00 to 3.91, based on the density of the solvent.
As the carbon-bearing gas solvent that is used in the foregoing method of manufacturing a magnetic recording medium, the following are mentioned: e.g., carbon dioxide CO2, ethane C2H6, and fluoroform CF3H.
More preferably, in the method of manufacturing a magnetic recording medium according to the second aspect of the invention, the step of preparing a lubricant which comprises fluoropolyether as the principal ingredient, represented by the chemical formula (1), comprises the steps of (a) letting fluoropolyether touch a carbon dioxide solvent which is placed in a supercritical or subcritical state through control of its temperature and pressure, thereby extracting fluoropolyether in the carbon dioxide solvent, (b) increasing in steps the density of the carbon dioxide which remains in the supercritical or subcritical state and includes the extracted fluoropolyether, and (c) serially extracting fractions of fluoropolyether separated, according to the carbon dioxide density increments, thereby adjusting alcohol hydration distribution which corresponds to difference between the greatest and smallest alcohol valences among the component molecules of the fluoropolyether to 0.48 or less and an average alcohol valence among the component molecules to fall within a range of 3.00 to 3.91.
In the step (c) of extracting serially extracting fractions of fluoropolyether in the step of preparing a lubricant, carbon dioxide in the supercritical state is used as its density changes within a range of 0.50 to 0.75 g/cm3.
According to a third aspect of the invention, the invention provides a magnetic recording apparatus comprising, at least, a magnetic recording medium for recording data thereon and a magnetic head which writes data on the magnetic recording medium, erases data from the magnetic recording medium, and reads data from the magnetic recording medium, wherein the magnetic recording medium is constituted by the foregoing magnetic recording medium according to the first aspect of the invention.